The main long-term objective of the proposed research is a physical-chemical understanding of the processes mediating excitation and adaptation in vertebrate photoreceptors. We propose to test the hypothesis that excitation is mediated by a diffusible intracellular transmitter substance that is liberated when rhodopsin light and that acts on specific ionic permeabilities of the plasma membrane. Specifically, we propose to test the hypothesis that the transmitter for excitation is intracellular ionized calcium by direct detection of light-induced changes of intracellular ionized calcium. We propose to do this by microspectrophotometry, using the metallochromic indicator dye, arsenazo III. We also propose to use voltage-sensitive dyes to detect changes of membrane voltage in vertebrate photoreceptor membranes. In addition, we propose to investigate involvement of cyclic nucleotides in the process of excitation and adaptation by means of intracellular electrical recording and direct intracellular injection of relevant substances through the recording micropipettes.